Water-soluble resin compositions comprising a water-soluble resin and a colorant are publicly known. For example, an aqueous ink comprised of a water-soluble resin and a colorant is being put to practical use as a substitute for an organic solvent-type ink having a problem of environmental pollution. However, the aqueous ink tends to exhibit a large viscosity change when it is stored for a long period, and give an image of poor reproducibility. Thus an aqueous ink exhibiting a reduced viscosity change when it is stored for a long period, and giving an image of good reproducibility is desired. Further, it is required that dot or fine line images developed therefrom have good reproducibility and exhibit good resistance to abrasion and mar. These desired characteristics are greatly influenced by the properties of a water-soluble resin contained in the aqueous ink.
An aqueous ink comprising a specific carbon black and a water-soluble resin having an acid value of at least 100 and a weight average molecular weight in the range of 3,000 to 7,000 in an aqueous medium is disclosed in Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) No. H4-18467. This aqueous ink exhibits reduced viscosity change when it is stored for a long period, but the resulting image has poor dot reproducibility and poor abrasion resistance.
JP-A H7-70254 discloses a water-soluble resin which is a copolymer prepared by copolymerizing 21% to 99% by weight of an ethylenically unsaturated carboxylic acid monomer with the remainder of other copolymerizable monomer in the presence of polyvinyl alcohol, and neutralizing the thus-obtained highly acidic copolymer with a basic compound. An aqueous ink comprising this water-soluble resin and a colorant exhibits a large viscosity change when it is stored for a long period, and gives images having poor abrasion resistance.